Abstract

Dorudon atrox is a medium-sized archaeocete cetacean, around five meters in length from snout to tail in life, and is known from the Birket Qarun and Gehannam Formations of Fayum, Egypt. These formations are primarily Bartonian, but probably straddle the Bartonian-Priabonian boundary. The Bartonian is the last European marine stage in the middle Eocene, while the Priabonian is the first marine stage in the late Eocene, and the formations from which D. atrox are recovered are around 41 to 40 million years old. Anatomical features found in D. atrox and its close relatives (Family Basilosauridae), indicate that these animals were the first fully aquatic cetaceans. The skull of D. atrox shows that the bones are arranged in a normal mammalian pattern and the skull lacks the cranial telescoping of either mysticetes or odontocetes. The external nares of D. atrox is retracted farther from the tip of the rostrum than in any known protocetid, but it is much farther forward than in any known neocetan. The cheek teeth of D.atrox (like those of all basilosaurids) bear multiple accessory denticles, a feature shared with both early mysticetes and early odontocetes. The vertebral column of D. atrox shows numerous adaptations to a fully aquatic existence including: compressed cervical vertebrae, non-revolute zygapophyses, relatively uniform size of posterior thoracics through anterior caudals, and dorso-ventrally flattened posterior caudal vertebrae. D. atrox, and fellow members of the Family Basilosauridae are the earliest cetaceans to show a dramatic increase in the length of the lumbar region due to the increase in the number of
lumbar vertebrae. The full vertebral count for D. atrox is 7 cervicals, 17 thoracics, 20 lumbars, no sacrals, and 21 caudals. The shapes of the caudal vertebrae indicate that although D. atrox had a fluke, it lacked a caudal peduncle like those seen in modem cetaceans. The forelimbs of D. atrox are modified into flippers. Movement at the elbow joint is restricted to the antero-posterior plane and pronation and supination at the elbow is not possible. The carpals lack saddle-shaped articular surfaces and are only capable of limited movement between adjacent bones. The hindlimbs of D. atrox are similar to those of Basilosaurus in that they are very reduced and could not support the body on land.
Reconstructions of the soft anatomy of Dorudon atrox, along with analyses of functional morphology show that D. atrox was a caudally propelled swimmer, much like modem cetaceans. Based on comparisons with modem cetaceans, D.afrox was a relatively slow sprint swimmer, with an estimated speed of around 2.4 body lengths or 12 meters per second. Estimates of the body mass of D. atrox, based on the total skeletal length show that D. atrox weighted around 2240 kg, slightly smaller than modem Ziphius cavirostris. The jaws and teeth, as well as some preserved stomach contents of D.atrox indicate that they captured individual prey items (fish, and possibly squid) with their anterior teeth, and then moved the prey items to the cheek teeth for oral processing. The brain mass of adult D. atrox was measured to be about 976 g, after correcting for possible overestimation of brain size due to the presence of a large vascular rete mirabile in the endocranial space. Using the estimated body mass and brain mass for D. atrox, an encephalization quotient of 0.425 was calculated. This indicates that D. atrox has a relatively small brain when compared to modem cetaceans of similar body size. Morphology of the lower jaw and ear region indicates that D. afrox, like most other archaeocetes could hear directionally under water. The anatomy of the periotic indicates that D. atrox could hear only low and mid range frequencies similar to those heard by modem mysticetes, and almost certainly lacked any echolocation capabilities like those of modern odontocetes. Phylogenetic analysis of Dorudon atrox, other archaeocetes, and early mysticetes and odontocetes indicates that D.atrox is close to the ancestry of modem cetaceans from the archaeocetes. Basilosaurids as a group share many features with early mysticetes and odontocetes including: accessory cusps on the teeth, greatly reduced hindlimbs, and an increase in the number of trunk vertebrae. D. atrox and its close relatives (Subfamily Dorudontinae) share all of these features with mysticetes and odontocetes while lacking the elongate trunk vertebrae of Basilosaurus, making them more likely to be the ancestors of modem cetaceans.